1. Field of the Invention
The present invention relates generally to the field of adhesive strength testing and in particular to testing the bonding strength of surface coatings and adhesives.
2. Description of the Prior Art
Three methods are conventionally used to compare or evaluate the adhesive strength of paint or other coatings to an underlying surface or substrate: ASTM D 2197-86, Standard Test Method for Adhesion of Organic Coatings by Scrape Adhesion; ASTM D 3359-87, Standard Test Methods for Measuring Adhesion by Tape Test; and ASTM D 4541-85, Standard Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers. Additionally, there is a conventional method of evaluation for comparing the adhesive strength of adhesive tapes: ASTM D 3330, Test Method for Peel Adhesion of Pressure-Sensitive Tape of 180 Degree Angle.
All of these methods are deficient in that they do not produce an independent standard of measurement for the adhesive strength. For many years those skilled in the art have attempted to find a suitable method for measuring adhesive strength which would provide results independent of the testing process. One such approach is the so-called "blister" method. In this process a fluid, either gas or liquid, is injected under the coating through a hole in its substrate forcing the coating to lift from the substrate in the form of a blister.
A promising enhancement of the blister technique was described by Dannenberg in 1960 (H. Dannenberg, "Measurement of Adhesion by a Blister Method", J. of Applied Polymer Science, Vol. V, No. 14, pp 125-134 (1961)). However, this process is complex and has not been widely accepted.
In 1987 Gent and Lewandowski described a further enhancement of the blister method for the evaluation of the adhesive strength of adhesive tape, but they reported observing several discrepancies in their results. (A. N. Gent and L. H. Lewandowski, "Blow-Off Pressures for Adhering Layers", J of Applied Polymer Science, Vol. 33, pp 1567-1577 (1987)). None of the known blister testing methods have been widely accepted.
One of the principal problems associated with the use of the blister method to evaluate the adhesion of a coating is that the film of the coating will often rupture before the coating debonds from the substrate. Also, the varying elasticities of most coatings will cause varying relationships between the height and the diameter of the paint blister formed, thus producing varying test results. Additionally, to determine bonding strength the blister method requires measuring both the elevation of the paint or other coating from the underlying surface to which it is bonded as well as the pressure at which debonding occurs. This results in a complex calculation. Further, the blister method for testing adhesive tapes used by Gent and Lewandowski has the problem of several unquantifiable variables in the process which lead to inaccurate results.
Another limitation common with conventional blister testing methods is that they must be conducted in a laboratory at normal room temperatures and are not suitable for testing at extreme environmental conditions, such as high or low temperatures.
What is needed is a relatively simple adhesive strength testing method and system which provide consistent measurements of bonding strength, which does not require measuring the elevation before separation, and which can be operated in extreme environmental conditions.